I use Apple Books app to read a PDF textbook. I highlighted a lot of stuff on around 500 pages of small font text.

I sync all my progress to iCloud. Yesterday, I turned off Wifi during reading to focus on studying.

Today, I opened the app and clicked on the book but it didn’t open. I tried troubleshooting for the whole afternoon but couldn’t get the file to open. I essentially have to reread 500 pages and highlight everything again.

Always, always back up your work.